Method of heat sealing polypropylene



United States Patent 3,380,870 METHOD OF HEAT SEALING POLYPROPYLENEHillard W. Ponncy, Jr., Somerset, N.J., assignor to Union CarbideCorporation, a corporation of New York No Drawing. Filed Sept. 4, 1963,Ser. No. 306,603 6 Claims. (Cl. 156308) The present invention relates toheat sealable coated polypropylene film and methods for heat sealingsaid film. More particularly the present invention relates to biaxiallyoriented polypropylene film coated with a polyvinyl acetal which coatingis not only heat scalable to itself but also to surface-treatedpolypropylene below the shrink temperature of such film.

Thin polypropylene films have a relatively low permeability to gases,vapors and liquids, and a relatively high degree of surface hardness,elasticity and abrasion resistance. Since these properties are highlydesired in a packaging film, it would be expected that these films wouldgain immediate acceptance in the packaging industry. Such a film,however, has one serious drawback: It exhibits low mechanicalproperties, such as tear resistance and impact resistance. Thisdeficiency has been overcome by monoaxially or biaxially stretching thefilm, thereby orienting it and improving these properties.

When such oriented film is heated to temperatures at or near the meltingpoint however, it shrinks, causing distortion. This shrinkage hasprecluded the use of the ordinary methods of heat sealing or weldingplastic film, as the fusion temperature of the film is above the shrinktemperature. This results in unsightly puckering and dis tortion aroundthe seal and weak areas which have become unoriented at and near theseal.

In accordance with the present invention it has been found thatheat-sealable biaxially oriented polypropylene film can be prepared byconditioning the surface of said film to impart wettability to a liquidhaving a surface tension of from 38 to 56 dynes/centimeter, and coatingsaid surface-conditioned film with a thin coating of a polyvinyl acetalhaving a softening point of from 49 to 97 C. While the thin coatingsgenerally have a thickness of from 0.05 to 2 mils, it has been founddesirable to use coatings having a thickness of from 0.1 to 0.4 mil. Thecoated films of this invention are clear and are readily heat sealed attemperatures well below the shrink temperatures of the film, to produceexceptionally strong seals.

Biaxially oriented polypropylene films are well known in the art as arethe methods for preparing them and surface conditioning them. Thesefilms are generally made from polypropylene resins having a density offrom 0.90 to 0.915 inclusive and a melt index of from 1 to 5 inelusive.

The film is biaxially oriented either during its manufacture orsubsequent thereto by simultaneously stretching the film in either orboth its longitudinal, or machine direction and its transversedirection. While the degree of stretch imparted to the film is generallydetermined by the use to which such film will be put, the film isusually stretched by a factor of from about 4 to about 6.

The oriented polypropylene film is then surface conditioned until it canbe wet by a liquid having a surface tension of from 38 to 56 dynes/cm.Suitable surface conditioning means are well known to the art as meansfor imparting printability to polyolefin films. Illustrative of saidsurface conditioning means are electrostatic discharge, especiallyelectrostatic corona discharge, flame treatment and oxidation bychemical agents such as chromic acid. All of these surface conditioningmeans can be used to impart wettability to the surface of polypropylenefilm, but electrostatic discharge treatment is ice preferred. Thisprocess generally comprises passing the polypropylene film between ahigh voltage potential discharge electrode and a ground rod. The voltageis adjusted to a sufilciently high level to discharge and preferablyform a corona discharge about the film; the film is then drawn throughthe discharge are gap at a rate sufficient to condition the surfaceuntil it can be Wet by a liquid having a surface tension of from 38 to56 dynes/ cm. and preferably from 44 to 46 dynes/cm.

The electrostatic treatment of such film is described in US. Patent3,018,189, issued on Jan. 23, 1962, to G. W. Traver and which is herebyincorporated by reference.

In the process of conditioning the surface of the film by flametreatment, the film is rapidly drawn through the flames of a gas burnerwhich is disposed to the width of the film.

The chemical treatment comprises contacting the film with concentratedchromic acid solution by immersion. In each instance the film is treateduntil the desired wettability has been obtained. For the purposes of thepresent invention polypropylene film which has been treated by exposureto corona discharge is preferred.

While many liquids can be used to conveniently determine the wettabilityof the surface of polypropylene film, a particularly useful liquidsystem consists of formamide and 2-ethoxyethanol for lower surfacetension ranges and formic acid and water for the higher ranges. Table Abelow indicates the composition of each mixture used to determinewettability and the surface tension of each.

TABLE A Formamide, Percent 2-methoxyethanol, Surface Tension,

by Volume Percent dynes/cm.

Formic Acid, Percent lVater, Percent Wetting Tension by 0 ume For thepurposes of the present invention it is preferred to utilize loweracetals which contain from two to 6 carbon atoms inclusive in the acetalportion of the condensate. These aldehydes condense with polyvinylalcohol through acetal formation of the aldehyde.

Illustrative of the structure of these condensates is O O O O I l R Rwherein R is an alkyl group of from 1 to 5 carbon atoms inclusive.

Illustrative of the aliphatic aldehydes which form acetal condensateswith polyvinyl alcohols and which having softening points of from 49 to97 C. are acetaldehyde, propionaldehyde, butyraldehyde,isobutyraldehyde, 2-ethylbutyraldehyde, and the like. Especiallysuitable as coat ing condensates in the present invention are polyvinylacetal, polyvinyl propional, polyvinyl butyral, polyvinyl isobutyral,and polyvinyl Z-ethylbutyral. Preferred as a polyvinyl acetal coating ispolyvinyl butyral as it exhibits a low softening point and highsolubility in alcohols.

In the practice of this invention a polyvinyl acetal is coated on thepolypropylene film by any convenient means. Illustrative of such meansare melt extrusion coating and solution coating. Solution coating,because of its convenience, is the preferred method of application. Inthis method the poly-vinyl acetal is dissolved in a suitable solventsuch as a lower aliphatic alcohol, for example methanol, ethanol,propanol, butanol, and the like, and the solution is applied to thesubstrate by any convenient coating means such as roller coating, brushcoating, dip coating, knife coating and the like. While theconcentration of the polyvinyl acetal in the coating solution is notcritical it has been found that solutions of from 1 to 20 percent byweight are convenient to coat.

The polyvinyl acetals employed in this invention are Well known in theart, generally being prepared by reacting a normally solid polyvinylalcohol with a lower aliphatic aldehyde.

The coated film is then dried by the evaporation of the solvent. Thecoating may be dried at room temperature or forced dried at elevatedtemperatures but below film shrink temperature. The polypropylene filmhaving a dried coating of polyvinyl alcohol/condensate in its surface isconveniently heat sealed to either another coated surface or to atreated but uncoated polypropylene surface. For example, if theelectrostatic discharge treated polypropylene film is coated only on oneside then the film can be sealed by pinch contacting the end portion ofthe coated film or by overlapping the edges. The portions of the film tobe sealed are held in intimate contact and raised to a temperature offrom 93 C. to 129 C. and preferably at temperatures of from 120 to 123C. This scaling is conveniently accomplished by utilizing a heat sealingdevice which clamps the film in intimate contact and heats the filmcontact portion by means of a heat sealing bar.

It has been found preferable to heat the film to a temperature of from120 to 123 C. to effect the heat seal as this temperature produces heatseals of exceptional peel and shear strengths.

In the example that follows the seal strength was evaluated by cuttingthe various films into samples 4 to 6 inches wide and sealing by meansof a bar type heat sealing apparatus. The specimens were cut into oneinch samples and placed in an Instron testing machine. Each seal wasevaluated for both peel strength and shear strength.

STANDARD TENSILE STRENGTH OF THE TACK SEALS The tack seal strength wasdetermined by tack sealing the specimens in 1 inch widths. Adjacent endsof the sealed films were mounted in the jaws of an Instron tensiletester having a cross head speed of 20 inches per minute to determinepeel strength. The same procedure was followed in determining sheartensile strength except that opposite ends of the opposing films wereplaced in the jaws in order to exert shear. The readings from the scaleof the tensile apparatus were recorded at point of delamination.

Example 1 One mil biaxially oriented polypropylene film treated byelectrostatic corona discharge to be wetted by a liquid having a surfacetension of 4446 dynes/cm. was coated with a 9 percent solution ofpolyvinyl butyral in n-propanol by roller coating. The coated film wasthen dried to a coating thickness of about 0.2 mil.

In a similar manner untreated biaxially oriented polypropylene film wast .en heat sealed using a standard bar sealer, at various sealing bartemperatures. All of the heat seals were effected under a pressure of 20psi. Variations of seals produced were treated and coated to treated andcoated polypropylene film; treated and coated to treated but uncoatedpolypropylene film; and treated and coated to untreated uncoatedpolypropylene film. The various seals effected were then evaluated inrespect to peel and shear.

The results of these evaluations appear in Table I below:

TABLE I.HEAT SEAL STREiIGTH OF POLYVINYL BUTYR AL COATED l BIAXIALLYORIENTED POLYPROPYLENE FILlVIS 1 Coated from 9% npropanol solution.

2 C/C=Surt'aces treated and coated.

C/'l= One surface treated and coated and one surface treated only. CIUT:One surface treated and coated and one surface untreated.

From the data of Table I above it can be clearly seen that biaxiallyoriented film can be easily sealed at temperatures below the shrinktemperature of the film (about 270 F.). It can also be seen from thisdata that stronger seals can be obtained at lower temperatures when twocoated and treated films are sealed than when only one film is coated.It is also indicated that stronger seals are produced in sealing singlecoated film to uncoated film when the uncoated film is treated to asuflicient level of wettability.

In a similar manner polyvinyl acetal, polyvinyl propional, polyvinylisobutyral, can be used as suitable coating materials to impart heatscalability to biaxially oriented polypropylene film.

What is claimed is:

1. The method of heat sealing biaxially oriented polypropylene film thesurface of which is wettable by a liquid having a surface tension offrom 38 to 56 dynes/ cm, which comprises (1) coating at least onesurface of said polypropylene film with a solution of a polyvinyl acetalin a solvent for said polyvinyl acetal, wherein said polyvinyl acetalhas a softening point of from 49 to 97 C. inclusive and wherein saidcoating has a dry thickness of from 0.1 to 0.4 mils, (2) drying saidcoating to remove said solvent and provide a coated surface, (3)contacting said coated surface with said polypropylene film and (4)heating the contacted surfaces to a temperature of from 93 to 129 C.

2. The method of claim 1 wherein the polyvinyl acetal is polyvinylbutyral.

3. The method of claim 2 wherein the solvent is npropyl alcohol.

4. The method of claim 1 wherein a sheet of the coated polypropylenefilm is heat sealed at the coated side thereof to an uncoated sheet ofsaid polypropylene film.

5. The method of claim 1 wherein two sheets of the coated polypropylenefilm are contacted at the coated surfaces thereof,

6. The method of claim 1 wherein the coated polypropylene film is heatsealed to itself through said coated surface.

(References on following page) Gaiser et a1. 161199 Watkins et a1.161-199 Lowry et a1. 161402 Hirt 117-138.8

Johnson 161199 Cupery 161-199 Schramrn.

Swerlick 117138.8 Traver 117-1388 Lake. Crooks et a1. 11713S.8

MORRIS SUSSMAN, Primary Examiner.

1. THE METHOD OF HEAT SEALING BIAXIALLY ORIENTED POLYPROPYLENE FILM THESURFACE OF WHICH IS WETTABLE BY A LIQUID HAVING A SURFACE TENSION OFFROM 38 TO 56 DYNES/ C.M., WHICH COMPRISES (1) COATING AT LEAST ONESURFACE OF SAID POLYPROPYLENE FILM WITH A SOLUTION OF A POLYVINYL ACETALIN A SOLVENT FOR SAID POLYVINYL ACETAL, WHEREIN SAID POLYVINYL ACETALHAS A SOFTENING POINT OF FROM 49* TO 97* C. INCLUSIVE AND WHEREIN SAIDCOATING HAS A DRY THICKNESS OF FROM 0.1 TO 0.4 MILS, (2) DRYING SAIDCOATING TO REMOVE SAID SOLVENT AND PROVIDE A COATED SURFACE, (3)CONTACTING SAID COATED SURFACE WITH SAID POLYPROPYLENE FILM AND (4)HEATING THE CONTACTED SURFACES TO A TEMPERATURE OF FROM 93 TO 129*C.